Wednesday, June 5, 2013

Words lead to confusion in phylogenetics

In 2008 Hanno Sandvik, from the University of Tromsø in Norway, made an interesting observation about word confusion and the history of phylogenetics:

In passing I would like to mention a speculation on the reason why phylogenetic systematics and cladistic methodology was rather quickly accepted in Germany (Ax 1977; Remane 1956; Schlee 1969), but provoked intense debates in English-language journals (verifiable with almost any issue of Systematic Zoology from the 1970s). I suspect that part of the problem was semantic. The German word for ‘relationship’ is "Verwandtschaft", but while the English word has all kind of abstract and symbolic connotations, including overall similarity, the German term is reserved for true, genealogical bonds (as is the Norwegian "slekt"). The statement that for instance the lungfish is more closely "verwandt" to the cow than to the salmon is quite uncontroversial in German. On the other hand, the statement that the lungfish is more closely related to the cow than to the salmon, was able to create a heated discussion — which was only peripherally concerned with the actual phylogeny of the groups concerned (Gardiner et al. 1979; Halstead 1978; Halstead et al. 1979).

This point seems to be directly relevant to the currently common uses of the expression "phylogenetic network", which can be seen to have an entirely homologous confusion. "Phylogenetic network" is used to describe both relationships based on general affinity (data-display or implicit networks) as well as the more restricted class of relationships based solely on genealogy (evolutionary or explicit networks). This duality potentially causes confusion every time the expression "phylogenetic network" is used, since it is not necessarily clear which meaning is intended. Fortunately, this has not (yet?!) lead to the sort of acrimonious "debates" that have been described for the introduction of cladistics to English-speaking biologists (Hull 1990; Felsenstein 2001).

This potential confusion also existed with the original introduction of what we would now call phylogenies, as this was done by French speakers — Buffon referred to his 1755 diagram as "une espèce d'arbre généalogique" [a type of genealogical tree], and Antoine Duchesne explicitly labelled his 1766 diagram as a "Généalogie". Similarly, Lamarck described his 1809 tree as "Servant à montrer l'origine des différens animaux" [Serving to show the origin of the different animals]. These are evolutionary diagrams, as their genealogical connotation is made clear. However, if this is translated into English as the generic word "relationship" then the clarity will disappear. [Tree-like diagrams produced by other authors at the same time usually referred to affinity, eg. "affinitates", "affinitatum".]

Incidentally, the word "phylogeny" is not now used with the original meaning given to it by Ernst Haeckel (Dayrat 2003). Haeckel used this word to describe a transformation series of character states, representing the order in which morphological changes have occurred through evolutionary time. These transformation series seem to owe more to Lamarck than to Darwin, as far as evolutionary ideas are concerned, but from them the tree of relationships could be constructed. For the tree itself Haeckel used the German word "stammbaum", which also has a directly genealogical meaning (eg. pedigree or family tree).